In the operation of a local exchange telephone network, it is desirable for a single central office to serve a large number of individual subscribers so as to achieve economies of scale. In the past, each subscriber served by a central office had to be directly connected thereto by a two-wire circuit. As a consequence, a large number of wire-pairs were required, increasing overall capital costs. In an effort to reduce the amount of cabling required to interconnect a large number of individual subscribers to a single central office, loop carrier systems have been developed to multiplex signals from the individual subscribers. A typical loop carrier system multiplexes signals from each active (e.g., off-hook) subscriber into a separate one of a plurality of time slots (channels) carried by a DS1 signal trunk connecting the loop carrier system to the central office. Since not all of the subscribers are active at any one time, the number of channels needed is usually a fraction (e.g., 1/2 or even 1/4) of the number of subscribers connected to the loop carrier system.
In the process of multiplexing the active subscriber signals onto the multi-channel DS1 signal trunk, each active subscriber signal, which is typically analog, is convened to a multi-bit digital channel. In the past, within the transmission equipment, each multi-bit digital signal had a specific bit dedicated to represent the parity of the signal. Thus, a check of the signal parity was accomplished in the equipment by determining the signal parity and comparing the value to that represented by the dedicated parity bit.
With the advent of Integrated Services Digital Network (ISDN) services, the signal from each active subscriber initially takes the form of a multi-bit digital signal, obviating the need for any signal conversion. Thus, the active-subscriber signal may be multiplexed directly onto the DS1 signal trunk. However, within the transmission equipment, the subscriber ISDN signals are not assigned a dedicated parity bit, making parity checking difficult.
Thus, there is a need for a technique for checking the parity of a digital signal within a transmission apparatus in circumstances where there is no reserved parity bit.